The present invention relates to superconducting solenoid installations, and particularly to a system for receiving cryogens when they must be removed from the vessels surrounding a solenoid.
The unique electrical properties of superconductors have led to proposals for storing large quantities of electrical energy in large superconducting coils, one type of which would be an annular solenoid installed, possibly below ground, so that its axis is vertical. The proposed solenoids would have a substantial axial height and diameter, the diameter possibly being of the order of 500 to 1000 meters or more.
The superconductive materials which are currently usable in practice, such as NbTi, must be maintained at a temperature in the vicinity of 1-4.degree. K in order to exhibit superconducting properties. Such temperatures can be established by surrounding the superconducting material with successive envelopes of cryogens having progressively higher boiling points.
For example, it is known to use, for this purpose, an envelope of helium, which can be made to have a boiling point lower than 4.degree. K., surrounded by an envelope of neon, having a boiling point of in the vicinity of 28.degree. K, the latter being surrounded by an envelope of nitrogen, having a boiling point in the vicinity of 77.degree.K. Each of these cryogens is maintained in the liquid state by a suitable refrigeration system.
During the operation of a superconducting device of the type described above, conditions can occur which result in a loss of superconductivity, accompanied by the generation of heat which causes the cryogens surrounding the solenoid to begin vaporizing. If this should occur, the cryogens must be purged as rapidly as possible in order to avoid a rapid, and possibly destructive, pressure increase.
In addition, there are occasions when the cryogens must be purged in order to permit maintenance or repair operations to be performed on the coil.
In either event, it is desirable to be able to save the cryogens in liquid form so that they will not be lost to the atmosphere or require an impractically large storage container.
It has previously been proposed to remove the helium employed in such a system to a storage tank located below the coil. This requires a relatively large conduit between the helium vessel surrounding the coil and the storage tank and involves an overall increase in the height of the installation. When the installation is to be installed below ground level, this entails an increase in the depth of the trench which must be dug to house the installation.
In addition, during cooling of the solenoid to the temperature required to establish superconductivity, the coil tends to undergo a significant contraction in the radial direction unless it is rigidly constrained. The cryogen vessels will also experience significant radial contractions. In the structures which have previously been proposed, movement of the helium vessel must be accompanied by movement of the storage tank and piping therebelow, which creates significant construction problems. Construction problems are complicated by the fact that at least the helium vessel must be supported by a structure surrounding the helium storage tank.